Bent‐core mesogens based on semi‐flexible dicyclohexylmethane spacers

New, bent‐core mesogens are described in which the core of the molecule is a semiflexible, di(4‐aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X‐ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered.     

Polycatenar Mesogens with Various Degree of Flexibility of Molecular Structure

A series of newly synthesised rod‐like polycatenar mesogens forms columnar phases, with the number of molecules in the column cross section depending on the core rigidity. For non‐symmetric molecules, an additional density modulation, namely helical arrangement of molecules with a periodicity of approximately 10 molecular distances develops along the columns. For one of the compounds, a new type of columnar liquid crystal phase with 3D positional order is observed. Introducing a stilbene unit in the mesogenic core enhances the fluorescent properties of the compounds. In the hexagonal columnar phase, polarised light emission is observed.     

Synthesis and columnar mesophase of fluorescent liquid crystals bearing a C2-symmetric chiral core

A chiral and fluorescent columnar mesogen prepared from chiral binaphthols is reported. This liquid crystal comprises a C2-symmetric chiral core with two staggered aromatic planes. Its hexagonal columnar (Col(h)) mesophase was characterized by appropriate physical methods.     

Calamitic or columnar mesomorphism determined by number and position of substituents in enaminoketone Cu(II), Ni(II) and Co(II) complexes

New enaminoketone metalomesogens with an almost equilateral triangular shape of the mesogenic core have been synthesized. They form either calamitic or columnar mesophases depending on the number and position of the substituted chains. The stability of the columnar as well as of the calamitic phases depends substantially on the central metallic ion.     

Bent-core mesogens based on semi-flexible dicyclohexylmethane spacers

New, bent-core mesogens are described in which the core of the molecule is a semiflexible, di(4-aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X-ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered.     

Ferroelectrically switchable columnar liquid crystal

The synthesis of a new disc-like mesogenic compound permitted, for the first time, ferroelectric electrooptical switching in a tilted columnar liquid crystal. The spontaneous molecular dipole moment is induced by the bend between the dibenzopyrene core and part of the eight O-hexyllactic acid chains attached to it. Two similar new compounds displayed a weak electrooptical effect in the columnar phase.     

Photoconductive Core–Shell Liquid‐Crystals of a Perylene Bisimide J‐Aggregate Donor–Acceptor Dyad

A novel core–shell structured columnar liquid crystal composed of a donor‐acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self‐assembles in solution into helical J‐aggregates guided by π–π interactions and hydrogen bonds which organize into a liquid‐crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs’ (J‐type) transition dipole moment parallel and the bithiophene side arms’ (H‐type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self‐assembled core–shell structure enables the implementation of the dye in two‐contact photoconductivity devices giving rise to a 20‐fold increased photoresponse compared to a reference dye without bithiophene donor moieties.     

Star-shaped oligobenzoates: non-conventional mesogens forming columnar helical mesophases

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non-conventional mesogens form various columnar mesophases over a broad temperature range. The liquid-crystal phases were characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and solid-state NMR spectroscopy. In addition to the high-temperature hexagonal columnar phases, the columnar self-assemblies undulate upon cooling and consequently form higher-ordered body-centred orthorhombic columnar 3D structures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low-temperature phases. Space filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star-shaped mesogens.     

Ferroelectrically switchable columnar liquid crystal

Abstract

The synthesis of a new disc-like mesogenic compound permitted, for the first time, ferroelectric electrooptical switching in a tilted columnar liquid crystal. The spontaneous molecular dipole moment is induced by the bend between the dibenzopyrene core and part of the eight O-hexyllactic acid chains attached to it. Two similar new compounds displayed a weak electrooptical effect in the columnar phase.     

Amphiphilic hairy disks with branched hydrophilic tails and a hexa-peri-hexabenzocoronene core

Amphiphilic discotic molecules with hydrophilic side branches consisting of hexaphenyl hexa-peri-hexabenzocoronene and hexabiphenyl hexa-peri-hexabiphenylcoronene as the aromatic core and hexa-substituted oligoethers as the branched peripheral chains have been synthesized, and their microstructure has been characterized. The discotic molecules based on dibranched oligoether side chains have been observed to self-organize into a well-ordered hexagonal columnar structure within liquid crystalline phases, which possessed an exceptionally high thermal stability and an unusually wide temperature range over >300 degrees C. We suggest that a combination of the large lateral dimensions of the rigid cores and disordered structure of the oxygen-containing branches tails is a driving force to the formation of a highly ordered columnar structure in the bulk state with enhanced molecular segregation. In contrast to the thermotropic phase behavior that favors the formation of highly ordered columnar aggregates through a strong stacking interaction, the hexabenzocoronene cores are packed in a face-on arrangement at the air/water interface and on solid surfaces with surface domains composed of an array of 7 x 7 molecules. We suggest a crablike molecular conformation and cluster-segregated monolayers with 6-fold symmetry and unusual face-on packing on a solid surface. Preliminary spectroscopic studies in the bulk state have shown that the molecules based on a hexaaromatic-substituted core may serve as functional supramolecular materials with high energy transfer characteristic within the columns due to near-perfect columnar ordering, which is unchanged over a wide temperature range. We believe that an absence of the crystallization phenomenon of side-branched oligoether chains is critical for the formation of long-range columnar ordering with strong intracolumnar correlation of conjugated disks important for high carrier mobility.     

Diketopyrrolopyrrole Columnar Liquid‐Crystalline Assembly Directed by Quadruple Hydrogen Bonds

A diketopyrrolopyrrole (DPP) dye self‐assembles via a unique hydrogen‐bonding motif into an unprecedented columnar liquid‐crystalline (LC) structure. X‐ray and polarized FTIR experiments reveal that the DPPs organize into a one‐dimensional assembly with the chromophores oriented parallel to the columnar axis. This columnar structure is composed of two π–π‐stacked DPP dimers with mirror‐image configurations that stack alternately through quadruple hydrogen bonding by 90° rotation. This exotic packing is dictated by the complementarity between H‐bonds and the steric demands of the wedge‐shaped groups attached at the core. This novel LC supramolecular material opens a new avenue of research on DPP dye assemblies with photofunctional properties tailored by H‐bonding networks.     

An investigation of the mesogenic properties of dibenzoquinoxaline derivatives

The syntheses of two new discotic molecules based on the dibenzoquinoxaline core are reported; comparison of their phase behaviors to a known dinitrile compound demonstrates the importance of specific functional groups in stabilizing columnar mesophases.     

Molecular dynamics and conformational behaviour of mesogenic resorcinarenes

The dynamic and conformational behaviour of four mesogenic resorcinarenes exhibiting columnar mesophases have been studied by a combination of broadband dielectric spectroscopy and deuterium solid state NMR. Broadband dielectric spectroscopy provided evidence for two relaxation processes present both in the mesophase and in the isotropic liquid. The high frequency process II, common to all mesogens, has been assigned to the libration of the carbonyl groups of the ester junctions between the core and the side chains. The low frequency process I, present in conformationally mobile mesogens 1 and 2, has been attributed to the ring inversion process of the macrocyclic core associated with dipole inversion along the columnar axis. Deuterium solid state NMR performed on 4, the deuteriated analogue of 1, confirmed the molecular dynamics attribution for process I, assigning the ring inversion to the interconversion of the two equivalent crown conformations.     

Nematic Triphenyltriazine Triesters and the Induction of the Columnar Mesophase by Fluorine Substitution

Whereas their para homologs are not mesogenic, the disk-shaped triphenyltriazine meta-trialkylesters obtained via trimerization of 3-cyanobenzoic alkylester, which are configurationally more flexible, exhibit a monotropic nematic mesophase. Introduction of fluorine atoms into the alkyl chains or into the phenyl moieties leads to the appearance of an enantiotropic columnar mesophase. If fluorine is introduced both in the chains and in the phenyl moieties, only a monotropic mesophase remains. Fluorination of either the alkyl chains or the aromatic core, but not both, appears thus as a simple means of inducing or stabilizing columnar self-assembly in disk-shaped systems. As the homeotropically alignable columnar mesophase can thus be made to persist at room temperature, as energies higher than 3 eV of the first excited triplet state are computed in agreement with the value reported for the parent arene, and as they are not fluorescent themselves, these compounds are of promise as aligning host matrices for blue-emitting TADF devices with improved light outcoupling. Dilution of a columnar with a nonmesogenic homolog induces the nematic state, indicating that the nanoscopic make-up of both mesophases is closely related.     

Structural factors controlling the self-assembly of columnar liquid crystals

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett sigma-values. The observed trends can be rationalized in terms of the strength of pi-pi interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.     

Influence of steric interactions and random side chain variations on the mesomorphic properties of bowlic mesogens

The effect of structural variations on the mesomorphic nature of columnar liquid crystals of general structure I is studied. X-ray crystal structure analyses of compounds la and 2a reveal the columnar organization of the macrocyclic cores in the solid state. The up and down asymmetry of the cores is resolved in an alternating sequence of up-up and down-down intermolecular contacts. Specific interactions between the R substituents are present, which influence the stacking distance between the cores in the down-down intermolecular contacts. Statistical incorporation of different side chains on the same macrocyclic core produces, when the difference in length between the alkyl chains is appropriate, random mixtures having wide mesomorphic ranges. The increase of disorder associated with the statistical side chain distribution on the core and with the structural heterogeneity of each component of the mixture have the effect of depressing mainly the crystal-mesophase transition temperature.     

Molecular dynamics and conformational behaviour of mesogenic resorcinarenes

The dynamic and conformational behaviour of four mesogenic resorcinarenes exhibiting columnar mesophases have been studied by a combination of broadband dielectric spectroscopy and deuterium solid state NMR. Broadband dielectric spectroscopy provided evidence for two relaxation processes present both in the mesophase and in the isotropic liquid. The high frequency process II, common to all mesogens, has been assigned to the libration of the carbonyl groups of the ester junctions between the core and the side chains. The low frequency process I, present in conformationally mobile mesogens 1 and 2 , has been attributed to the ring inversion process of the macrocyclic core associated with dipole inversion along the columnar axis. Deuterium solid state NMR performed on 4 , the deuteriated analogue of 1 , confirmed the molecular dynamics attribution for process I, assigning the ring inversion to the interconversion of the two equivalent crown conformations.     

Helical packing of discotic hexaphenyl hexa-peri-hexabenzocoronenes: theory and experiment

The arrangement of discotic hexa-peri-hexabenzocoronenes (HBCs) into columnar helical superstructures has been investigated in relation to their molecular architecture. The supramolecular structure of two hexaphenyl-substituted HBC derivatives, differing only in the chiral/achiral nature of the attached alkyl side chains, was studied by circular dichroism and temperature-dependent wide-angle X-ray diffraction on oriented filaments. A structural model in agreement with the experimental observations was developed on the basis of accompanying quantum-chemical calculations. The helical organization along the self-assembled columnar structures was induced by the steric requirements of the bulky phenyl rings near the aromatic core, i.e., by their rotation out-of-plane with respect to the aromatic core. On the other hand, a uniform handedness of the twist was generated by chiral alkyl substituents. At higher temperatures the degree of helical organization decreases due to lateral and longitudinal dynamics of the discotic molecules. Annealing at ambient conditions improved the long-range arrangement of the discs along the columnar structures. This reorganization indicated a self-healing of the plastic material which is desirable for application of discotics as active layers in electronic devices. The helical packing resulted in a considerable stability of the mesophase up to 500 degrees C, which has not been reported for a discotic so far.     

Formation of hexagonal columnar phases by heterocyclic pyrimidine derivatives

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds derived from pyrimidine as core group are reported. These compounds were prepared by condensation reactions of appropriate acetophenones and benzonitriles in the presence of trifluoromethanesulphonic anhydride. They were characterized by 1 H and 13 C NMR spectroscopy and elemental analysis, and their phase transitions characterized and studied by thermal analysis and polarizating microscopy. These compounds exhibit hexagonal columnar (Col h ) phases, as expected for disk-like molecules; the formation of columnar phases was found to be dependent on the numbers of alkoxy side chains attached. For those compounds having the same numbers of flexible side chains attached, the one with a preferred unsymmetric structure exhibited better mesomorphic properties. The observed improved mesomorphic behaviour of these compounds over other similar all-carbon heterocyclic compounds is attributed to the greater polarization of nitrogen atoms in the core ring.     

Theoretical Investigation of Macrodipoles in Supramolecular Columnar Stackings

Supramolecular columnar assemblies are known to form intrinsic macrodipoles, which play an important role in intercolumnar interactions and govern the self‐assembly on the mesoscale. A prominent class that provides this feature are trisamide derivatives, namely, 1,3,5‐benzenetrisamides and 1,3,5‐cyclohexanetrisamides. The understanding of how subtle changes in the chemical structure influence the columnar order and consequently the macrodipole formation is of fundamental interest. Here we report on the theoretical investigation of trisamide derivatives and how the formed macrodipole is related to the properties of the columnar aggregates. Calculations were carried out on a semiempirical level using the PM6 approximation, which is able to treat weak interactions like hydrogen bonding and dispersion forces with a sufficient accuracy. We have compared the influence of a benzene core with a cyclohexane core on the macrodipole formation. It was revealed that columnar aggregates based on 1,3,5‐cyclohexanetrisamides have much higher dipole moments than those formed with aromatic cores. A cooperative effect was found during aggregation, as longer aggregates show stronger hydrogen bonding, thereby facilitating the addition of the next molecule. We have also investigated the influence of the amide connection on the strength of the formed macrodipole. The trends observed for the macrodipole strength correlate with the calculated heat of formation. If the amide groups are inverted, the strength of the macrodipole is reduced and the negative heat of formation is increased. HOMO–LUMO gaps were correlated with the inverse of the dipole moment per monomer unit, thus indicating that the macrodipole might act as a perturbation to the supramolecular assemblies.